Microwave quasiparticle conductivity of LaFePO single crystals

We measured the temperature dependence of the microwave quasiparticle conductivity sigma(1) (T) in single crystals of LaFePO (T-c similar or equal to 7.5 K) to study the presence/absence of the coherence peak expected in s-wave superconductors. We use sensitive cavity resonators at several frequencies (15, 28, 60 GHz). We observe a broad enhancement of sigma(1) (T) below T-c which is almost independent of frequency. We find that such a frequency-independent enhancement is incompatible with the conventional coherence peak, and rather it can be explained by the rapid change of quasiparticle dynamics below T-c due to strong suppression of electron-electron inelastic scattering. (c) 2010 Elsevier B.V. AB rights reserved.